A fastener of this type, of particular application in spinal column surgery, is described in DE-U1 89.15,443.6. In essence that device consists of a lower part having the shape of a screw or a blade for attachment to a bone; and, adjoining the lower part, an upper part for fastening onto a rod. A channel that is open toward the top is formed in the upper part; this channel is bordered by two side walls between which the rod can be contained. Attachment of the rod within the channel is achieved by a threaded stud that can be screwed in. The lower end of this stud, which is meant to be engaged with the rod, is provided with attachment devices in the form of one or more sharp points.
The sharp points that are engaged during screwing in of the threaded stud between the two side walls of the body into the rod positioned there, dig at several locations into the surface of the rod and effect a stable attachment of the rod relative to the body. A disadvantage of this design, however, is that contact between the rod and the threaded stopper is at several separate locations, which can easily loosen if forces are exerted between the rod and the fastening device, as is common in the spinal column area. Forces which affect the rod act through long lever arms on the points which are engaged at separate locations. These lever arms can cause a sudden and irreversible loosening of fixation when very small increments of force are applied.
The invention provides a remedy for this problem by providing an osteosynthetic bone fastener of the type described, which will retain a tight clamping action when forces are applied, or else create a self-clamping effect so that loosening of the parts attached to each other is prevented.